1. Field of the Invention
This invention relates to an improved process of increasing gasoline octane number and total yield while also increasing operational flexibility in catalytic cracking units by the addition of an additive catalyst to conventional cracking catalysts.
2. Description of the Prior Art
Hydrocarbon conversion processes utilizing crystalline zeolites have been the subject of extensive investigation during recent years, as is obvious from both the patent and scientific literature. Crystalline zeolites have been found to be particularly effective for a wide variety of hydrocarbon conversion processes, including the catalytic cracking of a gas oil to produce motor fuels and have been described and claimed in many patents, including U.S. Pat. Nos. 3,140,249; 3,140,251; 3,140,252; 3,140,253; and 3,271,418. It is also known in the prior art to incorporate the crystalline zeolite into a matrix for catalytic cracking and such disclosure appears in one or more of the above-identified U.S. patents.
It is also known that improved results are obtained with regard to the catalytic cracking of gas oils if a crystalline zeolite having a pore size of less than 7 Angstrom units is included with a crystalline zeolite having a pore size greater than 8 Angstrom units, either with or without a matrix, see, e.g., U.S. Pat. No. 3,769,202. Although the incorporation of a crystalline zeolite having a pore size of less than 7 Angstrom units into a catalyst composite comprising a larger pore size crystalline zeolite (pore size greater than 8 Angstrom units) has indeed been very effective with respect to the raising of octane number, nevertheless it did so at the expense of the overall yield of gasoline.
In order to reduce automobile exhaust emissions to meet federal and state pollution requirements, many automobile manufacturers have equipped the exhaust systems of their vehicles with catalytic converters. Said converters contain catalysts which are poisoned by tetraethyl lead. Since tetraethyl lead has been widely used to boost the octane number of gasoline, refiners now have to turn to alternate means to improve gasoline octane number.
Many methods of octane improvement, however, reduce the yield of gasoline. With the present short supply of available crude oil and the concomitant high demand for unleaded gasoline with a sufficiently high octane number, refiners are faced with a severe dilemma. These trends are likely to continue in the foreseeable future.
One method of increasing octane number is to raise the cracker reactor temperature. This method, however, is very limited, since many units are now operating at maximum temperatures due to metallurgical limitations. Increasing the cracker reactor temperature also results in increased requirements for the downstream gas plant (i.e. gas compressor and separator). Since most gas plants are now operating at maximum capacity, any increased load could not be tolerated by the present equipment.
Improved results in catalytic cracking with respect to both octane number and overall yield are claimed in the process of U.S. Pat. No. 3,758,403. In said patent, the cracking catalyst was comprised of a large pore size crystalline zeolite (pore size greater than 7 Angstrom units) in admixture with ZSM-5 type zeolite wherein the ratio of ZSM-5 type zeolite to large pore size crystalline zeolite was in the range of 1:10 to 3:1.
The use of ZSM-5 type zeolite in conjunction with a zeolite cracking catalyst of the X or Y faujasite variety is described in U.S. Pat. Nos. 3,894,931; 3,894,933; and 3,894,934. The two former patents disclose the use of ZSM-5 type zeolite in amounts up to about 5 to 10 weight percent; the latter patent discloses the weight ratio of ZSM-5 type zeolite to large pore size crystalline zeolite in the range of 1:10 to 3:1.
The processes of U.S. Pat. Nos. 4,309,279 and 4,368,114 are predicated on the criticality of using only miniscule amounts of additive catalyst comprising ZSM-5 class zeolite to achieve improved results with respect to octane number and overall yield. In those processes 0.1 to 0.5% wt of ZSM-5 class catalyst gives the same beneficial results that were once thought obtainable only by adding much larger quantities of ZSM-5 class catalyst.
However, the ZSM-5 type zeolite catalyst, used as an additive catalyst in prior art cracking processes, was injected into the process at such locations that its residence time in the regenerator unit of the process was substantial. This, it is believed, contributed to a rapid aging of the ZSM-5 type zeolite, thereby necessitating frequent additions of substantial amounts of makeup additive catalyst. It is also believed that the circulation of the ZSM-5 type zeolite catalyst through the stripper and the riser mixing zone contributed substantially to the rapid deactivation of the additive catalyst.
It is a primary object of the present invention to decrease the extent of deactivation of the ZSM-5 type zeolite additive catalyst experienced in the prior art cracking processes.
It is an additional object of the present invention to decrease or substantially eliminate the circulation of the ZSM-5 type zeolite catalyst in the riser mixing zone, stripper and regenerator of the cracking reactor.
Other objects of the invention will be apparent to those skilled in the art from the following description of the invention and from appended claims.